During the game, you may come across an item called 'swim suit'. One is hidden on the field, and one is rewarded in the driller mini game, for beating mission 5 - attack of the killer bunnie.

when equipping the suit, character in battle, other than ziggy and kosmos, will change into their swim trunks/suit from the beach scene!
also, when beating a battle, the tech points are given a 25% boost. that means whoever is wearing the suit at the end of the battle will get 25% more tech points that usual. also, ziggy and kosmos can extract this ability, so they can use it too, although they both refuse to change clothes!

*eyes kosmos curiously*